"This is short."
Because c'est is ce+est. Ce is masculine while courte is feminine. It should be C'est court or cette est courte.
Because "C'est" is short for "Ça+est" (a compulsory contraction). "Ça" doesn't have a gender and is therefore always masculine. You would use "courte" if you had a word that is unequivocally feminine: "Cette jupe est courte" = "This skirt is short".
I disagree since "C'est une fille sympa" translates as "She is a nice girl". In this sentence, then, "C'est" clearly means "She is", not very masculine.
That's one of the main problems with grasping exactly what we mean by "c'est", because it is used in many different situations which all require different translations in English.
C'est is an impersonal way of describing almost anything, and can introduce a nominal group (like in your example, "une fille sympa" is a group with a noun) of any gender (you can absolutely say "c'est un garçon sympa", which would then translate to "he's a nice boy"). It would be the equivalent of saying in English: "That's a nice dog", just with an extended version that can also introduce a person in the same way. In both cases the adjective has to agree with the noun it follows or precedes.
However, when it introduces an adjective (like in the above example), then it's meant as "It's short" or "That's short", and can be expressed to describe anything, with "cela" or "ça" shortened to "c'" and replacing anything that was previously mentioned, and always impersonal, staying in the masculine singular form.
You also have to remember that our use of "court" never applies to the height of a person, for which we use the adjective "petit" (small). This sentence can therefore not be meant for a person, but rather relates to the length of something else.
According to about.com, it says that cela is used for the subject of verbs. It can also be C'est court as it says too. ça alone can't really mean it is o r this is and you wouldn't use ça est either. More info : http://french.about.com/od/grammar/a/indefinite-demonstrative-pronoun.htm
Because you can always say "Cela est", but we almost never use it. However, "Ça est" is a compulsory contraction, and is shortened to "c'est".
Difference between court and petit?
Additional info: I answered 'C'est petit' for 'This is short' and was marked correct.
'Petit' means small, so it would be something small in size, and 'court' means short, so it would be like an object. It's like the difference between small and short simply.
Because "bas" means low (as in, the opposite of "haut") while "court" means short (the opposite of "long").
Can "court" be used as it is sometimes in Spanish to mean that someone is not very intelligent? Could one say "Il/Elle is est court/e?
bas means low and court means short... I can see how in some cases it could be confusing... But think of low as referring to something big (opposite to high): ie low ceiling (it could also be figurative: ie low culture) Whereas short refers to the length of something/someone (opposite to long)... ie short hair (or figurative: short on cash)